In order to realize light emitting devices, such as light emitting diodes or laser diodes using group-III nitride-based compound semiconductors including GaN semiconductors, the structure and characteristics of an ohmic contact electrode provided between a semiconductor and an electrode pad are very important.
A currently available nitride-based light emitting device is formed on an insulating sapphire (Al2O3) substrate. Group-III nitride-based light emitting diodes formed on the insulating sapphire substrate are classified into top-emission type light emitting diodes and flip-chip type light emitting diodes.
The group-III nitride-based top-emission type light emitting diode outputs light, which is generated from a nitride-based active layer, through a transparent p-type ohmic contact electrode layer that makes contact with a p-type nitride-based cladding layer.
In addition, the top-emission type light emitting diode has poor electric characteristics, such as bad current injection and current spreading, derived from the characteristics of the p-type nitride-based cladding layer having a low hole carrier density value of 1018/cm3. Recently, a transparent current spreading layer having ohmic contact characteristics and superior electric conductivity is formed on the p-type nitride-based cladding layer so as to solve the problems of the nitride-based light emitting diode.
In general, a semi-transparent conductive thin film is extensively used as a current spreading layer having a p-type ohmic contact characteristic in the nitride-based top-emission type light emitting diode. Such the semi-transparent conductive thin film is obtained by combining a normal metal, such as nickel (Ni), with a noble metal, such as gold (Au), and then heat-treating the metal under a gas atmosphere having a pre-determined temperature.
As the semi-transparent conductive thin film is heat-treated, a preferred p-type ohmic contact electrode having a low specific contact ohmic resistance value of 10−3 to 10−4 ?cm2 can be formed. However, the p-type ohmic contact electrode has low light transmittance less than 80% in a blue light band of 460 nm. The p-type current spreading layer having low light transmittance absorbs most of light generated from the nitride-based light emitting diode, so the p-type current spreading layer is not suitable for the nitride-based light emitting diode having large capacity, large area and high brightness.
FIG. 1 is a sectional view showing a conventional flip-chip type nitride-based light emitting diode employing a reflective ohmic contact layer including a reflective metal layer formed on a p-type nitride-based cladding layer.
Referring to FIG. 1, the conventional flip-chip type nitride-based light emitting diode includes a substrate 110 on which a nitride-based buffer layer 120, an n-type nitride-based cladding layer 130, a multi quantum well nitride-based active layer 140, a p-type nitride-based cladding layer 150, and a p-type reflective ohmic contact layer 160 are sequentially stacked. The p-type reflective ohmic contact layer 160 is connected to a p-type electrode pad 170, and the n-type nitride-based cladding layer 130 is connected to an n-type electrode pad 180.
The p-type reflective ohmic contact layer 160 employs a high-reflective electrode material including aluminum (Al), silver (Ag) or rhodium (Rh) having superior light reflective characteristics. Since the above high-reflective electrode material has high reflectance, the p-type reflective ohmic contact layer 160 temporarily provides high external quantum efficiency (EQE). However, the high-reflective electrode material has a low work function value and creates new-phase nitride at an interfacial surface during the heat treatment process, so the p-type reflective ohmic contact layer 160 has a bad ohmic contact characteristic relative to the p-type nitride-based cladding layer 150 and represents bad mechanical adhesion and bad thermal stability, so that the life span of the semiconductor device is shortened and the productivity thereof is lowered.
That is, when depositing an aluminum reflective metal, which has a low work function value and creates new-phase nitride during the heat treatment process, on a p-type nitride-based semiconductor, a schottky contact causing serious voltage drop is formed at an interfacial surface between two materials, instead of an ohmic contact having a low specific ohmic contact value, so that the aluminum reflective metal is rarely adopted as a p-type reflective ohmic contact layer. Different from the aluminum metal, a silver metal makes the ohmic contact relative to the p-type nitride-based semiconductor. However, the silver metal exhibits thermal instability, bad mechanical adhesion relative to the nitride-based semiconductor, and great leakage current, so the sliver metal is not extensively used.
In order to solve the above problem, a p-type reflective ohmic contact layer representing low specific contact resistance value and high reflectance has been actively studied and developed.
FIG. 2 is a sectional view showing a conventional flip-chip type nitride-based light emitting diode employing a reflective ohmic contact layer including a conductive thin film formed on a p-type nitride-based cladding layer.
Referring to FIG. 2, in order to improve the interfacial characteristics between a reflective metal layer 260b and a p-type nitride-based cladding layer 250, a thin semi-transparent metal or transparent metal oxide layer 260a is formed on the p-type nitride-based cladding layer 250 as a p-type reflective ohmic contact layer 260 before the thick reflective metal layer is deposited. The p-type reflective ohmic contact layer 260 having the thin semi-transparent metal or transparent metal oxide can improve the electric characteristics, such as the ohmic contact characteristics, but the reflective ohmic contact layer that controls the optical performance of the flip-chip type light emitting diode has low light reflectance, so that the p-type reflective ohmic contact layer 260 has low EQE.
For instance, as shown in FIG. 2, Mensz et al. have suggested a dual-layered structure including nickel (Ni)/aluminum (Al), or nickel (Ni)/silver (Ag) through a document (electronics letters 33 (24) pp. 2066). However, the electrode structure of nickel (Ni)/aluminum (Al) may not constitute a preferred ohmic contact relative to a p-type nitride-based cladding layer, and the electrode structure of nickel (Ni)/silver (Ag) represents low reflectance causing low EQE due to a nickel metal inserted therein, although it may form a preferred ohmic contact relative to the p-type nitride-based cladding layer. Recently, Michael R. Krames et al. have suggested a multi-layered p-type reflective ohmic contact structure including nickel (Ni)/silver (Ag) or gold (Au)/nickel oxide (NiOx)/aluminum (Al) (US 2002/0171087 A1). However, such a multi-layered p-type reflective ohmic contact structure may cause scattered reflection at the interfacial surface between the multi-layered p-type reflective ohmic contact structure and the p-type nitride-based cladding layer, thereby lowering the EQE.
In addition, recently, a document [T. Margalith et al., Appl. Phys. Lett. Vol 74. p 3930 (1999)] discloses the use of transparent conductive oxide, such as indium tin oxide (ITO), having superior light transmittance than that of the nickel-gold structure employed as a conventional p-type multi-layered ohmic contact layer, in order to solve the problems of the top-emission type and flip-chip type light emitting diodes. A document (Solid-State Electronics vol. 47. p 849) shows that a top-emission type light emitting diode (TELED) employing the ITO ohmic contact layer represents improved output power than that of a TELED employing the conventional nickel-gold structure.
However, although the ohmic contact layer employing the above ITO ohmic contact layer increases the output power of the light emitting diode, the ohmic contact layer represents relatively higher operational voltage. This is because the ohmic contact layer has a relatively low work function value as compared with that of the p-type nitride-based semiconductor. For this reason, a high schottky barrier is formed at the interfacial surface between the p-type nitride-based cladding layer and the ITO ohmic contact layer, so that carrier injection is not readily achieved, thereby generating a great amount of heat and shortening the life span of the semiconductor device.